1. Field of the Invention
It is possible to use both pressure-controlled and stroke-controlled injection systems to supply fuel to combustion chambers of autoignition internal combustion engines. In addition to unit fuel injectors, these fuel injection systems are also embodied in the form of unit pumps and accumulator (common rail) injection systems. Common rail injection systems advantageously permit the injection pressure to be adapted to the load and speed of the engine. It is generally necessary to achieve the highest possible injection pressure in order to achieve high specific loads and reduce engine emissions of internal combustion engines.
2. Description of the Prior Art
DE 101 23 910.6 relates to a fuel injection system that is used in an internal combustion engine. Fuel injectors supply fuel to the respective combustion chambers of the engine. A high-pressure source acts on the fuel injectors; the fuel injection system designed according to DE 101 23 910.6 also includes a pressure booster with a moving pressure boosting piston that separates a chamber, which can be connected to the high-pressure source, from a high-pressure chamber connected to the fuel injector. The fuel pressure in the high-pressure chamber can be varied by filling a differential pressure chamber of the pressure booster with fuel or by emptying fuel from this pressure chamber.
The fuel injector has a moving closing piston for opening and closing injection openings oriented toward the combustion chamber. The closing piston protrudes into a closing pressure chamber so that fuel pressure can be exerted on it. This generates a force that acts on the closing piston in the closing direction. The closing pressure chamber and an additional chamber are comprised by a shared working chamber; all of the sub-regions of the working chamber are connected to one another continuously to permit the exchange of fuel.
With this design, triggering the pressure booster by means of the differential pressure chamber makes it possible to keep the triggering losses in the high-pressure fuel system low in comparison to a triggering by means of a working chamber that is connected to the high-pressure fuel source intermittently. In addition, the high-pressure chamber is only depressurized down to the pressure level of the common rail and not down to the leakage pressure level. On the one hand, this improves the hydraulic efficiency and on the other hand, it permits a more rapid pressure reduction down to the system pressure level so that intervals of time between the injection phases can be significantly shortened. In pressure-controlled injection systems equipped with a pressure booster, the problem arises that the stability of the injection quantities to be injected into the combustion chamber cannot be guaranteed, particularly the achievement of very low injection quantities such as those required in preinjections. This is primarily due to the fact that an injection valve member opens very quickly in pressure-controlled injection systems. As a result, very small variations in the triggering duration of the control valve have a powerful impact on the injection quantity. Attempts have been made to remedy this problem by using a separate needle stroke damper piston that delimits a damping chamber and must be guided in a high-pressure-tight clearance fit. This design does in fact permit a reduction in the needle opening speed, but it significantly increases the structural complexity and therefore the costs incurred to produce the injection system.
DE 102 29 418 has disclosed a device for damping the needle stroke in the fuel injector. In this design, the fuel injection system has a common rail, a pressure booster, and a metering valve. The pressure booster has a working chamber and a control chamber, which are separated from each other by an axially moving piston. A pressure change in the control chamber of the pressure booster causes a pressure change in a compression chamber that acts on a nozzle chamber via a fuel inlet. The nozzle chamber encompasses an injection valve member, which can be embodied, for example, in the form of a nozzle needle. A nozzle spring chamber that acts on the injection valve member can be filled on the high-pressure side from the compression chamber of the pressure booster via a line that contains an inlet throttle restriction. On the outlet side, the nozzle spring chamber is connected to a chamber of the pressure booster via a line that contains an outlet throttle restriction.
DE 102 29 415 likewise relates to a device for needle stroke damping in pressure-controlled fuel injectors. According to this design, a fuel injection apparatus includes a fuel injector, which can be acted on with highly pressurized fuel by a high-pressure source and can be actuated by means of a metering valve. The injection valve member is associated with a damping element, which can move independently of it and delimits a damping chamber. The damping element has at least one overflow conduit for connecting the damping chamber to an additional hydraulic chamber.
In the designs known from DE 102 29 418 and DE 102 29 415, the control valve is embodied in the form of a 3/2-way valve and controls a relatively large return quantity of the pressure booster. In particular, servo valves are used for this purpose. The above-mentioned triggering variants for fuel injectors equipped with only one valve have the disadvantage of a lack of flexibility with regard to the shaping of the injection pressure curve (rate shaping) in comparison to fuel injectors equipped with two actuators that are independent of each other.